Compressor

ABSTRACT

A rotary liquefied natural gas boil-off compressor has a series of compression stages. A gas passage passes through the series of compression stages. The gas passage extends through and is in heat exchange relationship with cooling means in the form of indirect heat exchangers. Each of the heat exchangers is cooled by LNG supplied from a pipeline. Flow control valves are provided for controlling the flow of LNG to the heat exchangers respectively. The valves are controlled in response to temperature sensors respectively, so as to maintain the inlet temperature of each of the compression stages at a chosen sub-ambient temperature or between chosen sub-ambient temperature limits.

CROSS-REFERENCE TO RELATED APPLICATIONS

National Stage application of International Application No.PCT/EP2005/000279 filed Jan. 13, 2005, which claims priority to BritishApplication No. GB 0400986.6 filed Jan. 16, 2004.

BACKGROUND OF THE INVENTION

This invention relates to a rotary liquefied natural gas boil-offcompressor. The invention also relates to a method of compressingboiled-off natural gas.

Liquefied natural gas is required to be stored in thermally-insulatedtanks. Notwithstanding the thermal insulation, there is always adiscernible inflow of heat from the surroundings which causes theliquefied natural gas to boil at a modest rate. The resulting boiled-offliquefied natural gas may be compressed and reliquefied or may be usedas a fuel. Use of the boiled-off natural gas as a fuel usually requiresits compression. For example, it has been proposed to use the boil-offgas from a shipboard liquefied natural gas storage tank to fuel a gasturbine forming part of the ship's propulsion system. Such a gasturbine, typically requires the boiled-off natural gas to be compressedto a pressure in the order of 20 to 40 bar. In another example, thenatural gas is employed together with diesel fuel in an engine employingboth fuels. In this example, the natural gas may be compressed to apressure in the range of 5 to 7 bar.

Conventional boil-off compressors employ six compression stages inseries if a pressure as high as 40 bar needs to be achieved. Thecompression of the gas in each stage generates heat. Accordingly thenatural gas is cooled between each pair of successive stages by indirectheat exchange with water. Such machines typically require quite largemotors and have a substantial power consumption.

BRIEF SUMMARY OF THE INVENTION

It is an aim of the present invention to provide a liquefied natural gasboil-off compressor which has a reduced size and power consumption.

According to the present invention there is provided a rotary liquefiednatural gas boil-off compressor having at least two compression stagesin series, a gas passage passing through the series of compressionstages, the gas passage extending through and being in heat exchangerelationship with at least one cooling means between the or each pair ofcompression stages, characterised in that the cooling means or at leastone of the cooling means is a cryogenic cooling means and in that thereis valve means for controlling flow of cryogenic coolant into thecryogenic cooling means in response to the inlet temperature, or arelated parameter, of the next compression stage downstream of thecryogenic cooling means so as, in use, to maintain said inlettemperature at a chosen sub-ambient temperature or between chosensub-ambient temperature limits.

The invention also provides a method of operating a rotary liquefiednatural gas boil-off compressor having at least two compression stagesin series and a gas passage passing through the series of compressionstages, the method comprising cooling the compressed boiled-off naturalgas by means of a cryogenic coolant downstream of one of the compressionstages and upstream of another, monitoring the inlet temperature, or arelated parameter, of the compressed natural gas at the inlet to theother compression stage, and adjusting the flow rate of cryogeniccoolant so as to maintain said inlet temperature at a chosen sub-ambienttemperature or between chosen sub-ambient temperature limits.

By use of a cryogenic coolant in accordance with the invention,particularly between each pair of successive compression stages, it ispossible to increase the ratio of the outlet pressure to the inletpressure of each such stage, and thereby typically reduce the number ofcompression stages required to achieve a particular pressure. Forexample, it is possible by means of the compressor and method accordingto the invention to raise the pressure of boiled-off liquid natural gasfrom 1 bar to approximately 40 bar using only four stages ofcompression, whereas a comparable conventional compressor employingnon-cryogenic cooling, typically water cooling, requires six stages toreach such a high pressure. As a result, the invention makes it possiblein these circumstances to achieve the same increase of pressure with asmaller machine using fewer compression stages and a lower powerconsumption.

The or each cryogenic cooling means may be either an indirect coolingmeans, e.g. separate passes of a heat exchanger, or a direct coolingmeans, e.g. the gas passage may extend through a chamber into which acryogenic liquid is introduced, for example, in the form of a spray. Itis preferred to have a cryogenic cooling means intermediate each pair ofcompression stages. If there are three or more compression stages it ispreferred that at least one of the cryogenic cooling means is anindirect cryogenic cooling means and at least one other is a directcooling means. In one preferred arrangement a cryogenic liquid is onlypartially vaporised in an indirect cryogenic cooling means and there isa passage placing the inlet of a direct cryogenic cooling means incommunication with the outlet of the indirect cryogenic cooling means.

There may also be a direct or indirect cryogenic cooling meansdownstream of the final compression stage. If indirect, the cryogeniccooling means may have an outlet communicating with an inlet to a directcryogenic cooling means upstream thereof.

The source of the cryogenic coolant is preferably the same liquefiednatural gas storage tank or array of storage tanks from which theboil-off gas is evolved. Such tanks are conventionally equipped withso-called stripping pumps which may be employed to supply the cryogenicliquid to the cryogenic coolant means. Alternatively, a dedicatedcryogenic coolant supply pump may be used.

There may be a cryogenic cooling means upstream of the first compressionstage. Such a cryogenic cooling means will not normally be operated asthe boiled-off natural gas is usually at a cryogenic temperature, butmay be required when the liquefied natural gas storage tank is nearlyempty, and the boil-off gas is therefore typically received at anundesirably high temperature, a condition that typically occurs after anocean going LNG tanker has discharged its load of LNG to a shore-basedterminal. The upstream cooling means may also be employed at start upwhen the piping is warm.

In order to supplement the rate at which natural gas is compressed, thecompressor according to the invention may have an intermediate inletcommunicating with a forcing liquefied natural gas vaporiser.

The forcing vaporiser and the cryogenic cooling means may if desiredshare a common pump for the supply of the cryogenic liquid.

The inlet temperature of each stage of the compressor is preferablymaintained at a temperature in the range of minus 50 to minus 140° C. Bythis means, it is possible to achieve a pressure ratio across each stagein the range of 2.15:1 to 3:1, and typically in the range 2.5:1 to 3:1.It is particularly desirable to avoid the presence of any droplets ofliquid in the natural gas entering any stage of the compressor.Accordingly, if any direct cryogenic cooling means is employed, theresulting cooled gas may be passed through an apparatus for disengagingparticles of liquid therefrom.

Compressors and methods of their use according to the invention will nowbe described by way of example with reference to the accompanyingdrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 5 of the accompanying drawings which are all schematic flowdiagrams, and

FIG. 6 shows a modification to any of the direct cooling stages of thecompressors shown in FIGS. 2, 4 and 5.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 of the drawings, there is shown an LNG storage tank2. For the purposes of ease of illustration, various pipes and valvesassociated with the tank 2, for example, its fill pipe and its LNGdischarge pipe, are not shown in FIG. 1 and the other drawings. Theconfiguration and operation of such LNG tanks is however well known inthe art. The tank 2 is typically located on board an ocean-going tanker(not shown). The tank 2 is shown containing a volume 4 of LNG. There isan ullage space 6 above the surface of the volume 4 of LNG in the tank2. The tank 2 is vacuum-insulated or has another form of thermalinsulation associated therewith so as to keep down the rate of flow ofheat from the ambient environment into the liquid 4. Since LNG boils ata cryogenic temperature, notwithstanding the thermal insulation of thetank 2, there is continuous adsorption of heat by the LNG from itssurroundings and hence continuous evaporation of the LNG into the ullagespace 6. As a result there is a continuous flow of vaporised gas out ofthe tank 2 into a passage 8. The passage 8 provides a flow of gas to aplural stage, centrifugal, natural gas compressor 10. Apart from thearrangements for cooling the gas downstream of each stage of thecompressor 10, and an optional arrangement for cooling the gas upstreamof the first stage thereof, it is essentially conventional but is madeof materials suitable for use at cryogenic temperatures. Unlike, say, anitrogen compressor, the natural gas compressor 10 is built to beexplosion- proof. Because many features of the natural gas compressor 10are conventional they are not illustrated in FIG. 1 of the drawings.Thus, for example, the rotary devices within the individual compressionstages are not shown.

The centrifugal compressor 10 has, as shown, four compression stages 12,14, 16 and 18 in series. The rotary member (not shown) of each of thecompression stages 12, 14, 16 and 18 may be mounted on the same driveshaft 20 and driven by an electric motor 22. It is not, however,necessary for all the compression stages to be mounted on the sameshaft. If desired, some of the stages may be mounted on a first shaftand others on a second shaft with drive from one shaft to another beingtransmitted through a gear box. Similarly, it is not necessary for anelectric motor 22 to be used to drive the shaft 20. Other kinds of motormay be used instead, or other forms of drive such as a steam turbine maybe used instead. If an electric motor 22 is employed, however, it ispreferably of a kind that has a single speed or it employs a frequencyconverter to enable the speed of rotation to be varied and thus theperformance of the compressor to be optimised.

Compression of the natural gas in each of the stages 12, 14, 16 and 18causes its temperature to rise. In general, the greater the temperaturerise the less thermodynamically efficient the compression. The greaterthe inlet temperature at each compression stage, the more power that isconsumed in compressing the gas. Further, as the temperature of the gasrises so its density falls. The more dense the gas, the smaller thecompression stage needed to achieve a given increase in pressure. Thiscorresponds to the enthalpy change at lower or higher temperature.

In accordance with the invention, a first cryogenic interstage heatexchanger 26 is located intermediate the first compression stage 12 andthe second compression stage 14; a second cryogenic interstage heatexchanger 28 is located intermediate the second compression stage 14 andthe third compression stage 16, and a third cryogenic interstage heatexchanger 30 is located intermediate the third compression stage 16 andthe final compression stage 18. The heat exchangers 26, 28 and 30 areemployed to effect cryogenic interstage cooling of the natural gas as itflows through the series of compression stages 12, 14, 16 and 18 insequence. A further cryogenic heat exchanger 32 is located downstream ofthe final compression stage 18 and a yet further cryogenic heatexchanger 24 is located upstream of the first compression stage 12 inthe sequence. Accordingly, the passage 8 extends in sequence through theupstream cryogenic heat exchanger 24, the first compression stage 12,the first cryogenic interstage heat exchanger 26, the second compressionstage 14, the second cryogenic interstage heat exchanger 28, the thirdcompression stage 16, the third cryogenic interstage heat exchanger 30,the final compression stage 18 and the downstream cryogenic heatexchanger 32.

Cooling of the boiled-off natural gas evolved from the volume of liquid4 in the tank 2 is effected in each of the interstage heat exchangers26, 28 and 30. The cooling is cryogenic so as to reduce the temperatureof the gas entering the next compression stage in the sequence to atemperature in the range minus 50 to minus 140° C. The upstream heatexchanger 24 may also be used to cool the gas to a similar temperatureupstream of the first compression stage 12, although, typically, the gasis already at this temperature so the heat exchanger 24 will duringnormal operation be by-passed or not operated. However, normal practicewhen discharging LNG from the tank 2 is to leave a small proportion ofthe liquid therein so as to maintain the tank 2 on the return journey.In consequence, during the return journey, the temperature of theboil-off gas tends to be much higher than when the tank 2 is full and itis then desirable to operate the upstream heat exchanger 24. Similarly,the downstream heat exchanger may be operated to cool the gas leavingthe final stage 18 of the compressor 10 to a temperature in the rangeminus 50 to minus 140° C. if the gas is required at such a cryogenictemperature. If it is required at ambient temperature, however, thecryogenic heat exchanger 32 may be omitted.

As shown in FIG. 1, all the cryogenic heat exchangers 24, 26, 28, 30 and32 are indirect heat exchangers. As will be discussed below, some or allmay alternatively be direct heat exchangers, that is to say heatexchangers in which the coolant fluid is mixed with the fluid to becooled. It should also be noted that it is possible in accordance withthe invention to operate only one of the interstage heat exchangers 26,28 and 30 cryogenically, but such a mode of operation is not preferred.It is also possible in accordance with the invention to provide a singleindirect heat exchanger unit that provides cryogenic cooling of theboiled off natural gas intermediate two or more pairs of compressionstages. For example, the heat exchangers 26, 28 and 30 may be combinedin a single unit. Further, if desired, the heat exchangers 24 and 32 maybe included in that unit.

The source of the cryogenic fluid for cooling the heat exchangers 24,26, 28, 30 and 32 is the storage tank 2 itself. A submerged pump 34within the tank 2 pumps liquefied natural gas (LNG) to a main pipeline36. The main pipeline 36 communicates with cooling passages in the heatexchangers 24, 26, 28, 30 and 32 by distributor pipes 38, 40, 42, 44 and46, respectively. Cooling is effected in each of these heat exchangersby partial or total vaporisation of the LNG in indirect heat exchangewith the boil-off gas being compressed. Flow of the LNG into each of theinterstage heat exchangers 26, 28 and 30 is controlled so as to maintainthe temperature of the boil-off gas at the inlet to the next compressionstage in the sequence at a chosen value or between chosen bounds. Thefirst interstage heat exchanger 26 has in the distribution pipe 40associated therewith a flow control valve 50 operatively associatedthrough a valve controller 70 with a temperature sensor 60 positioned inthe passage 8 at a region intermediate the exit for compressed boil-offgas from the heat exchanger 26 and the inlet to the second compressionstage 14. It can be readily appreciated by those well versed in the artof flow control valves that the valve 50 may be so arranged as tomaintain the temperature sensed by the sensor 60 at a chosen value, sayminus 130° C., or between chosen limits, say, minus 125° C. and minus135° C. Essentially identical flow control equipment is provided for thetwo other interstage heat exchangers 28 and 30 and for the upstream heatexchanger 24 and the downstream heat exchanger 32. Control of the LNGflow to the heat exchanger 28 is provided by a flow control valve 52 inthe distribution pipe 42. A temperature sensor 62 is positioned in thepassage 8 intermediate the outlet of the heat exchanger 28 and the inletto the next compression stage 16. A valve controller 72 is adapted toadjust the valve 52 so as to hold the sensed temperature at a chosenvalue or between chosen temperatures; an essentially identicalarrangement of flow control valve 54, temperature sensor 64 and valvecontroller 74 is provided for the third interstage heat exchanger 30; anessentially identical arrangement of flow control valve 56, temperaturesensor 66 and valve controller 76 for the downstream heat exchanger 32,and an essentially identical arrangement of flow control valve 48,temperature sensor 58 and flow controller 68 for the upstream heatexchanger 24.

Should the demand for LNG fluctuate, excess LNG may be returned to thetank via a return pipeline 78 which branches off from the main pipeline36. Preferably, there is a flow control valve 79 in the pipeline 78which is operatively associated via a valve controller 82 with apressure sensor 80 in the main pipeline 36 at a region of it upstream ofall the distributor pipes 38, 40, 42, 44 and 46 so as to maintain theLNG supply at constant pressure. If desired, the rate of pumping LNGfrom the tank 2 to the pipeline 36 may always be in excess of thatrequired for the purposes of the heat exchangers 24, 26, 28, 30 and 32so that there is always LNG returned to the tank 2 via the pipeline 78.Such return can be arranged to ameliorate or control the effects ofstratification of the LNG in the tank 2.

In operation of the apparatus shown in FIG. 1, LNG coolant passingthrough the heat exchangers 24, 26, 28, 30 and 32 flows in totally orpartially vaporised form to a main return pipeline 84 via pipes 86, 88,90, 92 and 94 associated with the heat exchangers 24, 26, 28, 30 and 32respectively. The pipeline 84 as shown returns the cold natural gas fromthese heat exchangers to the ullage space of the tank 6. Alternatively,the pipeline 84 may terminate in a region of the passage 8 upstream ofthe heat exchanger 24 or even downstream of the heat exchanger 32 if thepressure of the LNG supply is high enough.

The compressed boiled-off natural gas can, as stated above, be arrangedto leave the heat exchanger 32 at a temperature in the range of minus 60to minus 140° C. If the gas is to be reliquefied, then a lowertemperature is favoured. If the gas is, however, to be used as fuel forrunning an engine that provides propulsion for the ocean-going vessel,then a higher temperature is acceptable and indeed, if desired, thefinal heat exchanger 32 may be omitted or have a conventionalwater-cooled heat exchanger substituted for it.

The pressure ratio across each of the compression stages 12, 14, 16 and18 may be selected according to the required final outlet pressure. Fora gas turbine requiring a natural gas feed of 40 bar and a feed boil-offgas stream at a pressure of 1 bar, each compression stage may have apressure ratio of 2.6:1. If, however, the gas turbine requires naturalgas at a pressure of only 20 bar, the pressure ratio across eachcompression stage may be 2.1:1. One particular advantage of theinvention is that it is difficult to achieve pressure ratios as high as2.6:1 with natural gas when conventional cooling is used. If desired,the pressure ratio across each compression stage may be tailored byappropriately setting the inlet temperature to that stage.

Some of the possible alternative embodiments of the compressor accordingto the invention are shown in FIGS. 2 to 5 of the accompanying drawings.Like parts shown in FIGS. 1 to 5 of the drawings are indicated by thesame reference numerals.

Referring first to FIG. 2, the compressor and associated equipment showntherein, and their operation, are essentially the same as for thecorresponding compressor and equipment shown in FIG. 1 with theexception that the indirect heat exchangers 24, 26, 28, 30 and 32 ofFIG. 1 are replaced by direct heat exchangers 202, 204, 206, 208 and 210respectively. In each of the direct heat exchangers 202, 204, 206, 208and 210 the LNG is sprayed directly into the stream of boil-off gas fromthe storage tank 2 and therefore augments that stream. As a result,there is no vaporised natural gas to recirculate to the storage tank 2.Accordingly, the pipes 86, 88, 90, 92 and 94 and the return pipeline 84are omitted from the equipment shown in FIG. 2.

In a typical example of the operation of the apparatus shown in FIG. 2,the outlet pressures of the compression stages 12, 14, 16 and 18 are2.6, 6.3, 15.4 and 40 bar, respectively.

Whereas the boiled-off natural gas entering the pipeline 8 is depletedof heavier hydrocarbon impurities such as ethane, propane and butane,the LNG supplied by the pump 34 will normally contain these impurities.As a result, mixing of the boiled off natural gas with the LNG in thedirect heat exchangers 202, 204, 206, 208 and 210 will tend to raise thedewpoint of the boiled-off natural gas. It is thus desirable to ensurethat the temperature control is exerted so as to prevent particles ofliquid being carried from a mixing chamber into a compression stage. Ifdesired, the compressor shown in FIG. 2 can be provided with a devicefor disengaging liquid particles from the boiled off natural gasintermediate any chosen direct heat exchanger and the compression stageimmediately downstream thereof. For example, referring now to FIG. 6 ofthe drawings there is included between a mixing chamber 600 and thecompression stage 602 associated therewith a phase separation vessel 604having a demister pad 608 or the like inserted therein above an inlet606. The natural gas passes through the pad 608 and has any particles ofliquid disengaged therefrom. The phase separation vessel 604 has anoutlet 612 at its bottom for disengaged liquid. A flow control valve 614is located in the outlet 612 and may be arranged to open whenever thelevel of the liquid in the vessel 604 reaches that of a level sensor616. A valve controller 618 may be programmed so as to transmit thenecessary signals to the valve 614. The LNG may be returned to thestorage tank 2.

An arrangement such as that described above with reference to FIG. 6 maybe employed upstream of one or more of the compression stages shown inFIG. 2 of the drawings.

Another alternative to the equipment shown in FIG. 1 is illustrated inFIG. 3. The equipment shown in FIG. 3 is for use when the normalboil-off rate from the LNG stored in the tank 2 is too small to createenough energy for the ship's propulsion. For example, the boil-off maybe burnt in a gas turbine or injected into a dual fuel diesel engine.Typically, approximately 50% to 60% of the propulsion power can becovered by the natural boil-off gas. The rest of the propulsion power israised by oil or diesel fuel. The equipment shown in FIG. 3 can be usedwhen a greater proportion or all of the energy for propulsion of theship is to be generated by combustion of the gas. The equipment shown inFIG. 3 provides forced boiling in addition to the natural boiling of theLNG with the forced vapour being used for cryogenic cooling between atleast one pair of stages of the compressor.

The equipment shown in FIG. 3 adds to that shown in FIG. 1 a forcingvaporiser 302 which has an inlet for LNG communicating with the pipeline36 upstream of its union with the distributor pipe 38 and which also hasan outlet communicating with a region of the gas passage 8 intermediatethe outlet of the first compression stage 12 and the compressed naturalgas inlet to the first interstage heat exchanger 26. The forcingvaporiser 302 is used to augment the flow of compressed natural gas fromthe outlet of the first compression stage 12. The forcing vaporiser 302includes a shell-and-tube heat exchanger 304 in which liquefied naturalgas from the storage tank 2 is vaporised by indirect heat exchange withsteam or a hot mixture of water and glycol, and a mixing chamber 306 inwhich the thus vaporised natural gas is mixed with a flow of LNG whichby-passes the shell-and-tube heat exchanger 304. In order to supply theby-pass flow of LNG there is a by-pass passage 308 having a flow controlvalve 310 disposed therein. The flow control valve 310 is operativelyassociated with a valve controller 312 which receives signals from atemperature sensor 314 in a pipeline 316 extending from the outlet ofthe chamber 306 to the region of the gas passage 8 intermediate thecompression stage 12 and the first interstage heat exchanger 26. Thearrangement is such that the amount of by-pass LNG can be adjusted so asto maintain the temperature of the flow to the gas passage at a chosentemperature or between chosen temperature limits. The flow of LNG intothe shell-and-tube heat exchanger 304 is controlled by a flow controlvalve 318 which is operatively associated with a valve controller 320responsive to demand signals for extra gas from a pressure sensor 322 inthe pipeline 316 or downstream of the heat exchanger 32. The arrangementis such that the flow of vaporised natural gas from the forcingvaporiser 302 is at essentially the same pressure as the outlet pressureof the first compression stage 12.

Referring now to FIG. 4, there is shown a modification to the equipmentillustrated in FIG. 3, in which a combination of indirect and directheat exchangers is used. Thus, the indirect heat exchangers 24, 26 and28 shown in FIG. 3 are replaced by direct heat exchangers 402, 404 and406, respectively. Further, the pipes 38, 40, 42, 86, 88 and 90 arereplaced by partially vaporised natural gas recycle pipes 408, 410 and412 and the valves 48, 50 and 52, respectively are located in thesepipes. A further difference from the apparatus shown in FIG. 3 is thatthe forcing vaporiser 302 now communicates with a region of the gaspassage 8 intermediate the outlet of the second compression stage 14 andthe inlet to the direct interstage heat exchanger 406.

The operation of the equipment shown in FIG. 4 is similar to that shownin FIG. 3. In the heat exchangers 30 and 32 the LNG supplied to the heatexchangers 30 and 32 is only partially vaporised therein and theresultant mixture of cold vapour and liquid is recirculated to thedirect heat exchangers 402, 404 and 406. It is found that thisarrangement is particularly effective in keeping down the powerconsumption of the compressor 10.

Referring now to FIG. 5, the apparatus shown therein is essentially thesame as that shown in FIG. 2 but in addition employs a forcing vaporiser302 essentially the same as that shown in FIG. 3 except that itcommunicates with the gas passage 8 at a region thereof downstream ofthe outlet of the final compression stage 18 but upstream of the directheat exchanger 210. In the embodiment shown in FIG. 5, the forcingvaporiser 302 is operated so as to provide a flow of natural gas to thegas passage 8 at a pressure essentially the same as the outlet pressureof the final stage 18 of the compressor 10.

It can thus be appreciated that the compressor shown in FIG. 3 employs arelatively low pressure forcing vaporiser, whereas that shown in FIG. 5employs a relatively high pressure forcing vaporiser, and that shown inFIG. 4 employs a forcing vaporiser operating at a pressure between theoperating pressure of the other two compressors.

The compressors shown in FIG. 4 and FIG. 5 may be modified by having aphase separation vessel placed upstream of any stage thereof whichreceives directly cooled natural gas, the phase separation vessel beingessentially the same as the vessel 604 shown in FIG. 6 and being fittedwith a demister.

1. A rotary liquefied natural gas boil-off compressor having at leasttwo compression stages in series, a gas passage passing through theseries of compression stages, the gas passage extending through andbeing in heat exchange relationship with at least one cooling meansbetween the or each pair of compression stages, characterised in thatthe cooling means or at least one of the cooling means is a cryogeniccooling means and in that there is valve means for controlling flow ofcryogenic coolant into the cryogenic cooling means in response to theinlet temperature, or a related parameter, of the next compression stagedownstream of the cryogenic cooling means so as, in use, to maintainsaid inlet temperature at a chosen sub-ambient temperature or betweenchosen sub-ambient temperature limits.
 2. The compressor according toclaim 1, characterised in that the cryogenic cooling means comprises anindirect cooling means.
 3. The compressor according to claim 1,characterised in that the cryogenic cooling means comprises a directcooling means.
 4. The compressor according to claim 3, characterised inthat the direct cooling means comprises a chamber having an inlet forthe introduction of a cryogenic liquid.
 5. The compressor according toclaim 4, characterised in that the outlet of the direct coolingcommunicates with a vessel adapted to disengage particles of liquid fromthe natural gas, the vessel having an outlet for natural gascommunicating with said next compression stage.
 6. The compressoraccording to claim 1, characterised in that there is a cryogenic coolingmeans intermediate each pair of successive compression stages.
 7. Thecompressor according to claim 1, characterised in that there are atleast three compression stages in sequence and in that there is at leastone direct cryogenic cooling means and at least one indirect cryogeniccooling means.
 8. The compressor according to claim 7, characterised inthat an inlet of a direct cooling means communicates with an outlet ofan indirect cooling means.
 9. The compressor according to claim 1,characterised in that there is a cryogenic cooling means downstream ofthe final compression stage.
 10. The compressor according to claim 1,characterised in that there is a cryogenic cooling means upstream of thefirst compression stage.
 11. The compressor according to claim 1,characterised in that the compressor has an intermediate inletcommunicating with a forced liquefied natural gas vaporiser.
 12. Aliquefied natural gas storage tank having an outlet for boiled-offnatural gas communicating with a compressor as claimed in claim 1, thesaid cryogenic cooling means communicating with the liquefied naturalgas in the storage tank.
 13. A method of operating a rotary liquefiednatural gas boil-off compressor having at least two compression stagesin series and a gas passage passing through the series of compressionstages, the method comprising cooling the compressed boiled-off naturalgas by means of a cryogenic coolant downstream of one of the compressionstages and upstream of another, monitoring the inlet temperature, or arelated parameter, of the compressed natural gas at the inlet to theother compression stage, and adjusting the flow rate of cryogeniccoolant so as to maintain said inlet temperature at a chosen sub-ambienttemperature or between chosen sub-ambient temperature limits.
 14. Themethod according to claim 13, characterised in that the inlettemperature of each compression stage is maintained at a temperature inthe range of minus 50 to minus 140° C.
 15. The method according to claim14, characterised in that the pressure ratio across each compressionstage is in the range 2.15:1 to 3:1.
 16. The method according to claim15, characterised in that the pressure ratio across each compressionstage is in the range 2.5:1 to 3:1.